One of the most common strength training mistakes I see in the gym would be improper form during back exercises. Far too often I see people doing 'back exercises' without using their back at all. Pullups or rows using their arms and shoulders, without their shoulder blades moving at all. In the attached video, DIAKADI owner Billy Polson discusses some key stretches and technique pointers for making sure you are able to target and fully develop the muscles of your upper back, while maintaining proper posture and muscle balance for your upper body.
To find a large selection of advanced educational videos on exercise, nutrition, and lifestyle advice, check out the DIAKADI Youtube page (SFDIAKADITraining).